S=suma
o rezolvare prin metoda grafica:
daca un sfert din S=I...I atunci,
S=I....I...I.....I.....I
iar 3/4 din S=I....I......I.....I
3*S=3*S/4+180 adica
3*I..I...I..I...I=I.....I.....I....I+180
12*I....I=3*I....I+180
(12-3)*I....I=180
9*I.....I=180
I....I=180/9
I....I=20
S=I...I.....I...I....I=20*4=80 lei